Silicon nitride sintered body and process for producing the same

Abstract

Disclosed is a silicon nitride sintered body produced by subjecting a green compact of a mixed powder composed of 1) a silicon nitride powder having a percentage α crystallization of 93% or more and a mean grain diameter of 0.7 μm or less and 2) 5 to 15% by weight in total of a first sintering aid selected from among rare earth element, yttrium oxide and lanthanide oxides and a second sintering aid consisting of aluminum oxide or nitride and at least one selected from among oxides and nitrides of Mg, Ca and Li, to primary sintering in a nitrogen gas atmosphere under a pressure of 1.1 atm or less at 1500° to 1700° C.; and subjecting the sintered body to secondary sintering in a nitrogen gas atmosphere under a pressure of 10 atm or more at the primary sintering temperature or below, thereby giving a sintered body wherein the relative density of the sintered body is 99% or more and the precipitation ratio of an α-Si 3 N 4 (including β'-sialon) crystal phase ranges from 1:3 to 1:8 in terms of the peak intensity ratio in X-ray diffraction.

What is claimed is: 
     
       1. A silicon nitride sintered body produced by sintering a green compact of a mixed powder comprising 1) a silicon nitride source powder having a percentage α crystallization of 93% or more and a mean grain diameter of 0.7 μm or less and 2) a combined sintering aid composed of a first sintering aid which is yttrium oxide, and a second sintering aid consisting of aluminum oxide or nitride and at least one member selected from the group consisting of metal oxides and nitrides of Mg, Ca and Li, the total of said first sintering aid and said second sintering aid being not less than 5% by weight but not more than 15% by weight based on the total of said powder and said combined sintering aid, said sintering taking place in a nitrogen gas atmosphere at 1500° to 1700° C., wherein the precipitation ratio of an α-Si 3  N 4 , including α'-sialon, crystal phase to a β-Si 3  N 4 , including β'-sialon, crystal phase is from 1:3 to 1:8 in terms of the peak intensity ratio in X-ray diffraction, the weight ratio of said first sintering aid to said second sintering aid being 1.5:1 to 5:1 and the weight ratio of said aluminum oxide or nitride to said metal oxides and nitrides is 2:1 to 5:1. 
     
     
       2. A silicon nitride sintered body according to claim 1, which has a four-point bending strength of 100 kg/mm 2  or more as determined according to JIS R 1601 and a Weibull coefficient of 20 or more. 
     
     
       3. A silicon nitride sintered body according to claim 1, wherein the silicon nitride source is one produced by the imide decomposition method. 
     
     
       4. A silicon nitride sintered body according to claim 1, wherein the ratio of the first sintering aid to the second sintering aid is in the range of 1.5:1 to 5:1 in terms of percent by weight. 
     
     
       5. A silicon nitride sintered body according to claim 1, wherein in the sintered body, the mean grain diameter of the α-Si 3  N 4  including α'-sialon, crystal grain is 0.5 μm or less and the mean grain diameter of the β-Si 3  N 4  (including β'-sialon) crystal grain is 5 μm or less. 
     
     
       6. A process for producing a silicon nitride sintered body, comprising: subjecting a green compact of a mixed powder comprising 1) a silicon nitride source powder having a percentage α crystallization of 93% or more and a mean grain diameter of 0.7 μm or less and 2) a combined sintering aid composed of a first sintering aid which is yttrium oxide and a second sintering aid consisting of aluminum oxide or nitride and at least one member selected from the group consisting of metal oxides and nitrides of Mg, Ca and Li, the total of said first sintering aid and said second sintering aid being not less than 5% by weight but not more than 15% by weight based on the total of said powder and said combined sintering aid to primary sintering in a nitrogen gas atmosphere under a pressure of 1.1 atmosphere or less at a primary sintering temperature of 1500° to 1700° C. so that the relative density of said sintered body is at least 93%; and   subjecting said sintered body to secondary sintering in a nitrogen gas atmosphere under a pressure of at least 10 atmosphere at said primary sintering temperature or below, whereby said sintered body was a relative density of at least 99% and the precipitation ratio of an α-Si 3  N 4 , including α'-sialon, crystal phase to a β-Si 3  N 4 , including β'-sialon, crystal phase is from 1:3 to 1:8 in terms of the peak intensity ratio in X-ray diffraction.